Dielectrically-loaded multifilar antennas are disclosed in Published International Patent Application No. WO 2006/136809, British Patent Publication No. 2442998A, European Patent
Publication No. EP1147571A, British Patent Publications Nos. 2420230A, 2444388A, 2437998A and 2445478A. The entire disclosure of these patent publications is incorporated in the present application by reference. Such antennas are intended mainly for receiving circularly polarised signals from a Global Navigation Satellite System (GNSS), e.g. from satellites of the Global Positioning System (GPS) satellite constellation, for position fixing and navigation purposes. Other satellite-based services for which such antennas are useful include satellite telephone services such as the L-band Inmarsat service 1626.5-1675.0 MHz and 1518.0-1559.0 MHz, the TerreStar S-band service, the ICO Global Communications S-band service and the SkyTerra service. The S-band services have allocated frequency bands in the range of from 2000 MHz to 2200 MHz. The reader would appreciate that TerreStar and ICO are both owned by Dish Network, and SkyTerra was acquired by Harbinger Capital Partners and became part of LightSquared in 2010.
Each of these antennas has a plurality of helical antenna elements which are plated on a substantially cylindrical electrically insulative core made of a high relative dielectric constant material such as barium titanate. The material of the core occupies the major part of the volume defined by the core outer surface. Extending through the core from one end face to an opposite end face is an axial bore or passage containing a feed. At one end of the bore conductors of the feed are coupled to respective antenna elements which have associated connection conductors plated on the respective end face adjacent the end of the passage. At the other end of the passage, one of the feed conductors is connected to a conductor which links the antenna elements and, in each of these examples, is in the form of a conductive sleeve encircling part of the core to form a balun. Each of the antenna elements terminates on a rim of the sleeve and each follows a respective helical path from its connection to the feed.
The conductive sleeve referred to above is coupled to the outer shield of the feed structure where it emerges at a proximal end face of the antenna to form a balun at the frequencies of certain modes of resonance of the antenna. This effect occurs when the electrical length of the sleeve and its connection to the feed structure (with respect to currents on the inner surface of the sleeve) is (2n−1){circle around (2)} g/4 where ){circle around (2)} g is the guide wavelength of the relevant resonance, and n is a positive integer. The operation of the conductive sleeve rim as a resonant element is described in more detail in the above-mentioned EP1147571A.